1. Field of the Invention
The present invention relates to a projector for projecting a picture on a screen, and more particularly to a reflection type overhead projector.
2. Description of the Related Art
Heretofore, there is known a reflection type overhead projector in which a Fresnel concave mirror is utilized. The term of a Fresnel concave mirror means a mirror which is formed in such a manner that a usual concave mirror is cut into separate concentric annular parts each having a fine width, and these annular parts are rearranged on the same plane, the mirror having a planar plate shape in the entire configuration but serving to function as a concave mirror. Hereinafter, what is referred to as a Fresnel mirror represents a Fresnel concave mirror. The foregoing projector is so arranged that a Fresnel mirror is fitted to the upper surface of a unit body with the mirror surface thereof facing upwardly. A post is erected at one upper corner of the body. A transparency put on the Fresnel mirror is irradiated using a light source within a projector head attached to the upper end of the post, and the reflected light from the transparency is projected on a screen through a projecting lens within the projector head and a reflecting mirror. The most valuable advantage of the projector of this type is in making it possible to employ a transparency of relatively large size. While production of a small-sized transparency must be relied on (the photographic technology), a large-sized transparency can be produced by printing with ink, handwriting or other alternatives to photographic technology. A large-sized transparency is subjected to correction or revision with little difficulty. Furthermore, a large-sized transparency has another advantage, which cannot be otherwise obtained; a particular point may be indicated on a viewing screen, by pointing a small stick to the corresponding location on the transparency.
However, use of a large-sized transparency necessarily requires an increased distance between a plate on which the transparency is put and the projecting lens. The aforesaid post is employed to make the unit lighter while maintaining a sufficient distance between the plate and lens. This arrangement presents the problem that, because the projector head which incorporates a light source and the projecting lens are supported at the upper end of the post. The optical axis may easily be misaligned even with a slight shift of the post. Hence, the transparency cannot be correctly projected on the screen. Although a projector of a type which is provided with a foldable post so as to reduce the unit size has been proposed, such an arrangement is susceptible to misalignment at the joint portion between the post and the body, thus resulting in optical axis misalignment.